The invention relates to helical scan video recorders adapted for reproducing video signals modulated on a high-frequency carrier and fixed on at least one track of an elongated magnetic tape that is wrappable helically about a rotatably mounted drum.
In arrangements of this type, at least one magnetic pick-up head that is rotatable in a scanning path on the periphery of the drum detects video signals that are modulated on an associated track of the tape as the tape is longitudinally advanced. In order to faithfully reproduce the video signal in the playback head, it is important to assure that each video track be brought into alignment with the scanning path of the associated magnetic pick-up head. This is accomplished by initially providing an oscillatory scan of each head with respect to the track, thereby imparting a corresponding cyclic variation in the video output from the track. Such video output variation may then be employed as a tracking control signal for application to a tape velocity regulating system, which in turn varies the velocity of the tape advance in a direction to correct any detected misalignment of the track and the head.
In one particularly advantageous arrangement of this type, the scanning operation is initiated by applying, to the tape drive servo, a sub-audio train of repetitive pulses to effect a generally triangular or sinusoidal wave modulation of the component of tape velocity parallel to the drum axis. Such cyclic modulation is superimposed upon a DC position error signal derived from the detected video envelope. At the conclusion of the predetermined interval normally required for the tape drive servo to adjust the tape track to its optimum position, the cyclically modulated error voltage is disconnected from the servo input, and a quiescient signal ideally of the appropriate magnitude necessary to maintain the tape in its optimum position is thereafter applied to the servo input.
One disadvantage of the last-mentioned arrangement is that the forced cyclic oscillation applied to the tape servo during the scan operation requires that at least several periods of such oscillation of the composite error signal picked up by the head must elapse before such composite signal is reduced to a value that is near optimum for servo track adjustment. The consequent predetermined interval between the start and termination of the scanning mode must accordingly be set at a relatively large value, so that the response of the overall track alignment operation is relatively slow. In addition, the circuitry and mechanisms necessary for the generation and application of the above-mentioned triangular or sinusoidal wave modulation of the tape advance velocity are complicated and expensive.